1. Field of the Invention
The present invention relates to a focal length detector of a camera having a zoom lens.
2. Description of the Related Art
A focal length detector of a camera having a zoom lens which is provided therein with a code plate, having predetermined code patterns, and a brush, whose sliding contacts are in sliding electrical contact with the code plate to detect the focal length of the zoom lens, is known. In this known type of focal length detector, the brush and the code plate move relative to each other during zooming according to the movement of a moving barrel of the zoom lens so that the focal length of the zoom lens is detected by detecting a combination of electrically-connected or disconnected code patterns (electrical-connection indication signals or zoom code data) via the brush. For instance, in cameras having a zoom lens whose moving barrel or barrels are driven to retract from a wide-angle extremity to even a housed position in the camera body when power of the camera is switched OFF, the housed position, the wide-angle extremity, the telephoto extremity and intermediate positions (different focal-length positions) between the wide-angle extremity and the telephoto extremity of a moving barrel of the zoom lens are each detected. In such cameras, the detectable number of the focal lengths between the wide-angle extremity and the telephoto extremity can be increased to substantially an infinite number in theory by counting two-bit repetitive patterns. However, in practice, the detectable number of the focal lengths is limited by the size of terminals of the code plate and the amount of relative movement between the code plate and the brush. In the case of a focal length detector in which one particular sliding contact of the brush and a corresponding particular conductive pattern of the code plate which remain in contact with each other are used as terminals for ground, the code plate needs to be provided with at least three conductive patterns including the aforementioned conductive pattern (i.e., two conductive patterns in the form of a two-bit repetitive pattern and a pattern for ground), or the brush needs to be provided with at least three sliding contacts including the aforementioned sliding contact, in order to obtain two-bit repetitive codes. In order to detect specific focal length positions such as the housed position, the wide-angle extremity and the telephoto extremity, the code plate further needs to be provided with at least one more conductive pattern or the brush further needs to be provided with at least one more sliding contact to obtain electrical-connection indication signals or zoom code data that are different from the aforementioned two-bit repetitive codes. Therefore, in such a focal length detector, the code plate or the brush needs to be provided with at least four conductive patterns or at least four sliding contacts, respectively, which are to be electrically connected to a focal length detecting circuit (a control circuit) of the camera. In such a conventional focal length detector, regardless of which of the code plate and the brush is electrically connected to the focal length detecting circuit, the brush is generally provided with four sliding contacts. However, from a view point of miniaturizing the camera including the focal length detector, preferably the number of sliding contacts of the brush is as small as possible.